The present invention relates to regenerated, high molecular weight, thermoplastic polyurethane resins and to a process for regenerating thermally degraded and/or off-grade, thermoplastic polyurethane resins.
One of the most significant problems related to the thermoplastic polyurethane technology, either thermoplastic polyurethane engineering plastics or elastomers, is the large amount of waste materials. These waste materials comprise either thermally degraded resins, scrap or misformulated resins. These polymeric materials, which have a low molecular weight, heretofore could not be regenerated and were normally considered waste material. However, it has become common practice, for example, to chop scrap into small pieces and blend them back into a virgin resin at appropriate ratios. Only a limited amount, typically 10 percent, of this regrind can be used without any severe property losses.
The problem associated with using the regrind together with a virgin resin in most thermoplastic materials is that the resulting resin will have a broader molecular weight distribution than the virgin resin. This broader molecular weight distribution causes it to have inferior properties as compared to the virgin resin.
Still another problem is that the higher the molecular weight of a thermoplastic polyurethane resin, the better its physical properties. However, the higher the molecular weight the more processing difficulties appear, such as, to cause a high melt viscosity during processing. Thus, the conventional molecular weight range for thermoplastic polyurethanes varies between 125,000 and 350,000. Upon using this molecular weight range, parts possessing acceptable physical properties are prepared without undue processing difficulties.
It would, therefore, be most desirable, from an economic, environmental and commercial standpoint, to provide a process for regenerating higher molecular weight thermoplastic polyurethane plastics and elastomers with a narrow molecular weight distribution and good physical properties from degraded or off-grade thermoplastic polyurethane resins, which do not present the processing difficulties inherent to high molecular weight resins.